Passage 6
Until recently astronomers have been puzzled by the fate of red giant and supergiant stars. When the core of a giant star whose mass surpasses 1.4 times the present mass of our Sun (M⊙) exhausts its nuclear fuel, it is unable to support its own weight and collapses into a tiny neutron star. The gravitational energy released during this implosion of the core blows off the remainder of the star in a gigantic explosion, or a supernova. Since around 50 percent of all stars are believed to begin their lives with masses greater than 1.4M⊙, we might expect that one out of every two stars would die as a supernova. But in fact, only one star in thirty dies such a violent death. The rest expire much more peacefully as planetary nebulas. Apparently most massive stars manage to lose sufficient material that their masses drop below the critical value of 1.4 M⊙before they exhaust their nuclear fuel.
Evidence supporting this view comes from observations of IRC+10216, a pulsating giant star located 700 light-years away from Earth. A huge rate of mass loss (1 M⊙ every 10,000 years) has been deduced from infrared observations of ammonia (NH3) molecules located in the circumstellar cloud around IRC+10216. Recent microwave observations of carbon monoxide (CO) molecules indicate a similar rate of mass loss and demonstrate that the escaping material extends out-ward from the star for a distance of at least one light-year. Because we know the size of the cloud around IRC+10216 and can use our observations of either NH3 or CO to measure the outflow velocity, we can calculate an age for the circumstellar cloud. IRC+10216 has apparently expelled, in the form of molecules and dust grains, a mass equal to that of our entire Sun within the past ten thousand years. This implies that some stars can shed huge amounts of matter very quickly and thus may never expire as supernovas. Theoretical models as well as statistics on supernovas and planetary nebulas suggest that stars that begin their lives with masses around 6 M⊙ shed sufficient material to drop below the critical value of 1.4M⊙. IRC+10216, for example, should do this in a mere 50,000 years from its birth, only an instant in the life of a star.
But what place does IRC+10216 have in stellar evolution? Astronomers suggest that stars like IRC+10216 are actually “protoplanetary nebulas” –old giant stars whose dense cores have almost but not quite rid themselves of the fluffy envelopes of gas around them. Once the star has lost the entire envelope, its exposed core becomes the central star of the planetary nebula and heats and ionizes the last vestiges of the envelope as it flows away into space. This configuration is a full-fledged planetary nebula, long familiar to optical astronomers.
27. The primary purpose of the passage is to
(A) offer a method of calculating the age of circum-stellar clouds
(B) describe the conditions that result in a star’s expiring as a supernova
(C) discuss new evidence concerning the composition of planetary nebulas
(D) explain why fewer stars than predicted expire as supernovas
28. The passage implies that at the beginning
of the life of IRC+10216, its mass was approximately
(A) 7.0 M⊙ (B) 6.0 M⊙ (C) 5.0 M⊙ (D) 1.4 M⊙
29. The view to which line 18 refers serves to
(A) reconcile seemingly contradictory facts
(B) undermine a previously held theory
(C) take into account data previously held to be insignificant
(D) resolve a controversy
30. It can be inferred from the passage that the author assumes which of the following in the discussion of the rate at which IRC+10216 loses mass?
(A) The circumstellar cloud surrounding IRC+10216 consists only of CO and NH3 molecules.
(B) The circumstellar cloud surrounding IRC+10216 consists of material expelled from that star.
(C) The age of a star is equal to that of its circumstellar cloud.
(D) The rate at which IRC+10216 loses mass varies significantly from year to year.
31. According to information provided by the passage, which of the following stars would astronomers most likely describe as a planetary nebula?
(A) A star that began its life with a mass of 5.5 M⊙, has exhausted its nuclear fuel, andhas a core that is visible to astronomers
(B) A star that began its life with a mass of 6 M⊙, lost mass at a rate of 1 M⊙ per 10,000 years, and exhausted its nuclear fuel in 40,000 years
(C) A star that has exhausted its nuclear fuel, has a mass of 1.2 M⊙, and is surrounded by a circumstellar cloud that obscures its core from view
(D) A star that began its life with a mass greater than 6 M⊙, has just recently exhausted its nuclear fuel, and is in the process of releasing massive amounts of gravitational energy
32. Which of the following statements would be most likely to follow the last sentence of the passage?
(A) Supernovas are not necessarily the most spectacular events that astronomers have occasion to observe.
(B) Apparently, stars that have a mass of greater than 6 M⊙ are somewhat rare.
(C)Recent studies of CO and NH3 in the circumstellar clouds of stars similar to IRC+10216 have led astronomers to believe that the formation of planetary nebulas precedes the development of supernovas.
(D) Astronomers have yet to develop a consistently accurate method for measuring the rate at which a star exhausts its nuclear fuel.
33. Which of the following titles best summarizes the content of the passage?
(A) New Methods of Calculating the Age of Circumstellar Clouds
(B) New Evidence Concerning the Composition of Planetary Nebulas
(C) Protoplanetary Neula: A Rarely Observed Phenomenon
(D) Planetary Nebulas: An Enigma to Astronomers
Passage 7
Noses have it pretty hard. Boxers fatten them. Doctors rearrange them. People make jokes about their unflattering characteristics. Worst of all, when it comes to smell, no one really understands them.
Despite the nose’s conspicuous presence, its workings are subtle. Smell, or olfaction is a chemosense, relying on specialized interactions between chemicals and nerve endings. When a rose, for example, is sniffed, odor molecules are carried by the rising airstream to the top of the nasal cavity, just behind the bridge of the nose, where the tips of the tens of millions of olfactory nerve cells are clustered in the mucous lining. The molecules somehow trigger the nerve endings, which carry the message to the olfactory lobes of the brain. Because smell information then travels to other regions of the brain, the scent of a rose can elicit not only a pleasurable sensation but emotions and memories as well.
Though just how odors stimulate the nerves is unknown, scientists do know that our sense of smell is surprisingly keen, capable of distinguishing up to tens of thousands of chemical odors. The laboratory task of isolating the components of an odor is far from simple. Tobacco smoke, for example, is made up of several thousand different chemicals. Moreover, smell researchers must grapple with the problem of what to call the different odors that the nose detects. People generally refer to smells by their sources of associations. Descriptions such as “like a wet dog” or “like my elementary school” may convey perceptions but are vastly inadequate for labeling the chemistry involved.
To further complicate research, olfaction is connected to other sensations. Besides olfactory nerves, the nasal cavity contains pain-sensitive nerves that perceive sensations such as the kick in ammonia of the burning in chili peppers. Smell also interwines with taste to create flavor. A coffee drinker holding his nose while sipping would taste only the bitter in his brew, for taste receptors generally appear limited to bitter, salty, sour and sweet. The sense of smell is ten thousand times more sensitive than taste and makes subtle distinctions among lemon, chocolate, and many more flavors.
So how does the nose manage this sophisticated discrimination? Lake of evidence hasn’t kept scientists from speculating. One idea is that every odor molecule vibrates at its own frequency, creating patterns of disturbance in the air similar to the wave patterns produced by sound. According to this theory, the nerves act as receivers for the unique vibrations of every odor molecule. The scheme requires no direct contact between the molecule and the nerve cell.
Another suggestion is that primary odors, equivalent to the primary color s of vision underlie all smells and are detected by receptor sites on the olfactory nerves. Different combinations of about thirty basic smells, with labels such as malty, minty, and musky, could form an infinite number of odors.
Other scientists think that each smell is its own primary smell. They believe the olfactory nerve endings have specific receptor proteins that bind to each of the chemicals people can sense. This theory, however, calls for thousands of different proteins – none of which has been found.
“The science of smell is so empirical, ”says Robert Gesteland, a neurobiologist at Northwestern University, “there’s no predictive base for experiments.” Unlike the senses of sight, touch, and hearing, olfaction studies have attracted only a small share of scientific interest. That may change. Researchers hope that unraveling the mystery of smell and taste disorders that affect two million Americans. And in the future, with enough known about smell, it might be possible to endow strange, unappealing but nutritious foods with more familiar odors, perhaps expanding the world’s food supply. For the moment, however, what the nose knows it isn’t revealing.
34. We may conclude from this passage that
(A) our sense of smell is as important as any of our other senses
(B) each smell is its primary smell
(C) olfactory study has become a major research area
(D) there is much more to be learned about the nose
35. According to the passage the only statement which is not true is
(A) doctors use smell research to better understand taste disorders
(B) significant progress has been made in separating the various proteins in the nerve endings
(C) smell researchers have difficulty in labeling different odors
(D) our sense of taste is not nearly as acute as our sense of smell
36. Which of the following sentences from the passage illustrates the need for further research?
(A) Smell also interwines with taste to create flavor.
(B) The molecules somehow trigger the nerve endings, which carry the message to the olfactory lobes of the brain.
(C) The science of smell is so empirical, there’s no predictive base for experiments
(D) Smell, or olfaction, is a chemosense, relying on specialized interactions between chemicals and nerve endings
37. In attempting to analyze the intricacies of smell discrimination, some scientists have suggested
I. that odor molecules work in the same way that sound waves do
II. that primary odors, which are inherent in all smells, are communicated to receptor sites on the olfactory nerves
III. that recognition takes place as the molecule stimulates the nerve cell
(A) II only (B) I and II only (C) I and III only (D) I, II, and III
38. The author attempts to lighten this serious biological report by means of
(A) the incongruity of widespread smell research
(B) similes such as “like a wet dog”
(C) the opening and closing statements
(D) the confession of our basic ignorance
39. The comparison of a smell to a person’s elementary school was made in order to
(A) illustrate a unique perception
(B) show how imagery may be employed in a lab situation
(C) point out the uselessness of such a description to scientists
(D) personalize a complicated topic
(E) maintain the reader’s interest
40. According to the passage, we can find massive quantities of olfactory nerve cells
(A) in every chemosense (B) on the brain lobes
(C) behind the bridge of the nose (D) in special taste receptors
42. The broadest example of a major problem facing smell researchers is contained with
(A) the reference to tabbacco smoke (B) the reference to the rose
(C) the coffee drinker’s experience (D) Robert Gesteland’s statement